If you want to create a presentation to illustrate, say, a new idea or project, then you'd probably consider using a presentation package such as PowerPoint first.
But there are actually some great products available on the web which are even more powerful and, for the most part, totally free.
For example, check out Prezi at www.prezi.com. It offers loads of free templates that you can use or edit, to create presentations that are truly unique. The screen shot below isn't particularly impressive, for sure, but I put it together in about 90 seconds.
A free account at Prezi gives you 100 MB of storage space and unlimited presentations, although you'll need to take out a paid membership if you don't want all your work to be publicly viewable.